A shot sleeve is a device for injecting molten metal into a die or mold. Relatively simple in construction, it typically comprises a metal cylinder defining an axial chamber and a piston fitted within the chamber to act as an injection ram. An aperture in the side of the sleeve opens into a portion of the cylinder chamber just in front of the piston when it is in the rest position. This portion of the chamber is called the "well" and the molten metal is poured into the well for temporary residence before the piston is actuated.
Because of the high temperature difference between the molten casting metal and the elements of the shot sleeve, useful life expectancy of prior art devices is quite short. This is believed to be due in part to warpage and erosion of the axial chamber, and resulting piston wear. The surface of the bore opposite the well is subjected to the highest temperature. By the time molten metal has entered the rest of the bore, it has cooled and is much less damaging. The temperature differential from the top to the bottom in a horizontal sleeve creates warpage in the sleeve.
This problem of warpage and erosion is exacerbated when the shot sleeve is used at higher casting rates or with metals having a high melting point. Thus, while aluminum has a lower melting point than, for example, steel or iron, aluminum has a much higher rate of heat transfer (approximately five times as high as steel or iron). Moreover, the cycle for casting aluminum is much shorter than that for steel or iron and, consequently, many more pounds of aluminum may be cast per hour than is the case of other metals. Hence, many more BTU's are transferred per hour to the shot sleeve than is generally encountered in casting of ferrous metals. For example, while iron or steel is generally cast at a temperature of approximately 3000.degree. F., the quantities are small, one or two pounds, and the interval between shots is much longer (as much as 5 to 10 times) compared to aluminum where cycle times allowing as little as 30 seconds between shots is not uncommon.
Since the casting cycle is shorter and the heat transfer rate is higher, conducting the damaging heat away from the pour hole area must be done in a much shorter time, and thus, efficient heat transfer is very important.
Efforts have been made to increase the useful life of a shot sleeve by a variety of methods, such as water or air cooling the sleeve itself. In U.S. Pat. No. 3,533,464 to Parlanti, a plurality of radially extending, heat dissipating fins are disposed about the periphery of the injection chamber of the shot sleeve. U.S. Pat. No. 3,515,203 to Parlanti et al., discloses a laminated injection cylinder particularly useful for die casting high temperature molten metals, such as iron or steel. An inner sleeve formed of a super alloy is surrounded by an intermediate layer of beryllium copper alloy, which is in turn enclosed by an outer shell comprised of heat treated H-13 steel. The intermediate beryllium copper layer extends all the way to the die end of the shot sleeve chamber. This laminated shot sleeve relies solely on heat transfer from the inner layer to the outer layer by the intermediate layer to prevent excessive heat build-up in the well area of the shot sleeve.
U.S. Pat. No. 3,672,440 to Miura et al. discloses an injection cylinder useful in the die casting of ferrous and other metals of high melting points. The injection cylinder, which is inclined with respect to the horizontal, comprises an outer cylindrical sleeve of high heat conductivity and an inner cylindrical lining which is removably fitted in the outer sleeve. The inner lining includes a plurality of cylindrical sections of short axial length which are clamped together. If any of the sections become warped or otherwise damaged during molding operations, the damaged section may be removed and replaced.
While the systems disclosed by Parlanti et al. and Miura et al, may be useful for casting metals having high melting points, neither system is totally satisfactory for use in casting metals having low melting temperatures, such as aluminum or magnesium. The beryllium copper layer in the Parlanti et al. device extends all the way to the die end of the shot sleeve, this end of the sleeve is held in place by the die. The end of the sleeve fitted into the die expands as heat is absorbed by the steel sleeve. Typically, an aluminum biscuit, which cushions the impact of the piston as the piston packs the molten metal into the die, forms at the die end of the sleeve. Hence, during rapid cycling, the two ends of the shot sleeve are heated at a much faster rate than the middle of the sleeve. The sleeve at the die end of Parlanti et al.'s sleeve would expand much more than the middle, causing the fit between the piston and the sleeve to change drastically.
Similarly, the short cycle time and high heat transfer typical of aluminum casting negates the usefulness of Muira's shot sleeve when applied to low melting point metal with high heat transfer capability, such as aluminum or magnesium casting. Since the beryllium copper sections comprising the inner liner are removable, they necessarily cannot be fitted tightly within the outer shell. The rate of transfer of heat from the hot inner sleeve to the outer shell is, necessarily, compromised.
In my U.S. Pat. No. 4,623,015 I disclose an improved shot sleeve for molding molten metals which has a surface pattern of copper welded to the outside of the metal body of the shot sleeve. The spiral pattern of the welded copper is designed to passively convey heat away from the well area.
While the device disclosed in my above-cited U.S. patent has found some commercial acceptance, it has certain limitations. Since the copper is disposed on the outside surface of the shot sleeve, it is relatively far away from the hot metal and heat transfer is, thus, impaired. Certain shot sleeves which are components with standard die casting machines have sleeve walls of a relatively great thickness, thus exacerbating the problem.
Another major advance in shot sleeve construction is disclosed in my U.S. Pat. No. 4,926,926. There, a unique shot sleeve construction is presented which is based on a three-layer body wall construction which has the ability to rapidly dissipate a great amount of heat from the central chamber or inner barrel of the shot sleeve. As noted above, such temperature control is needed to prevent warpage which can interfere with movement of the plunger or piston within the shot sleeve central bore to the point where the shot sleeve cannot charge the die to which it is coupled.
While the shot sleeve construction of U.S. Pat. No. 4,926,926 has proven highly successful, the die casting art continues to move in the direction of faster cycling, which in turn presents increased problems of thermal strain and wear. Accordingly, I have continued to experiment with thermal control within the shot sleeve charging bore, and have now discovered a cooling arrangement which provides even more rapid heat transfer from the metal charging bore.